There are TWO actual problems with the Air2:
1) It IS hard to share (but I always share my primary anyhow).
2) If you must disconnect the hose, you've lost power BC inflation, as well as the backup reg.
However, for some of us, there are advantages that outweigh these:
1) They ARE less bulky to pack (we always carry-on our regs)
2) There is one less hose to deal with in the water
3) You ALWAYS know where the backup reg is
4) You are USED to handling the backup reg
5) The backup reg is less often in an inaccessible position
6) They are reliable, at any reasonable depth (I?ve used mine at 160+, but that is another discussion)
My wife and I bought Air2's the month they came out. We have used them all over the world. We are STILL using the original units (gee, they might be ?18 years old now??).
I believe that the major factor in why they are so often maligned is simply that they are not, and never will be, COOL.